[Un article de The Conversation écrit par Thibault Leroy – Biologiste, chercheur en génétique des populations, Inrae & Jérémy Clotault – Enseignant-chercheur en génétique, Université d’Angers]
The XIXᵉ, a Rosomanian century
Since antiquity, roses have been cultivated, both in China and in the Mediterranean region, for their medicinal properties, in particular the anti-inflammatory and anti-microbial properties of essential oils or as sources of vitamin C in rushorhodons (roses' false-fruits), and for their strong symbolic, especially religious, However, for centuries, the number of varieties has remained very limited, around a hundred. The XIXe century marks a major turning point for European horticulture with an excitement carried by a new enthusiasm for the aesthetics of roses. The collectors – whose most emblematic figure was probably the Empress Joséphine de Beauharnais – and the creators of new French varieties had a decisive role in this boom, giving birth to a real “Rosomania” and contributing to an explosion in the number of varieties, going from a hundred to almost 8,000 varieties! As a comparison, since this period, the number of varieties has certainly continued to progress, standing around 30,000 varieties today, but at a less sustained rate of growth.
Beyond the increase in the number of varieties, the XIXe century was marked by a great diversification of the characteristics of roses. The number of petals, in particular, has become a criterion of major interest. Botanic roses, ancient cultivated forms directly from nature, generally only had five petals. Over the 19the A century, the horticultural selection made it possible to obtain varieties with much more sophisticated flowers, some of dozens, even hundreds of petals. However, this development did not follow a linear progression: the first half of the century saw a clear increase in the number of petals, marked by a fashion of the hundred-feuilles roses, while the second half was rather characterized by stagnation, even a return to simpler forms. Certain varieties very worked on the aesthetic level have thus been selected to paradoxically have only five petals.
The biggest difference between the roses at the start and the end of the 19th centurye lies in a fundamental character: the feedback. Until the middle of the 19th centurye A century, the roses were mainly non-backing, that is to say that they only flourished once a year, in the spring. The ability of roses to flow, generating new flowers during the summer, or even until the fall, is not the result of chance! This characteristic was an important objective of the selectors of the time. This story, very associated with the genetic crosses carried out, especially with Chinese roses, has left a lasting imprint, both in our contemporary gardens and in the very genomes of roses.
Although they were not aware of it, the selectors were also able to contribute to the introduction of unfavorable characters. Thus, by studying the levels of symptoms of the disease of black spots on hundreds of varieties of the XIXewe have highlighted an increase in the sensitivity of varieties. This disease is today considered one of the first causes of phytosanitary treatments on roses, which is not without asking health questions about the exposure to pesticides of florists and other professionals in the horticultural sector. Our study nevertheless found genomic regions associated with resistance to this disease, offering the hope of a selection to new, more resistant varieties.
Our study has not only focused on predisposition to diseases but also to one of the most important characteristics of roses: their odor. The scent of roses is explained by a complex cocktail of fragrant molecules. Two molecules are nevertheless very important in what is called the smell of ancient rose, geraniol and 2-phenyléthanol. We have studied the scent of hundreds of varieties and observed a very high variability of it, as much as a composition. However, our results do not support, or extremely marginally, a reduction in the scent of roses during the XIXe century. The loss of perfume has probably arrived later, during the XXe Century, a period which saw the appearance of a specific varietal creation activity for cut flower roses and which would have neglected the scent of roses in favor of the duration of vase, allowing to relocate production in countries with reduced production costs.
Roses with mixed genomes
To better understand the origin and diversity of these 19th century rosesewe must now dive into the universe of infinitely small: that of their genomes. In our new study, we have undertaken to characterize the genetics of more than 200 varieties in detail, by relying on tens of thousands of markers, that is to say of targeted information on particular areas of genomes, known as variable according to the varieties, distributed over all of their chromosomes. For around thirty varieties, we went even further, by deciphering all of their genome, providing no longer tens of thousands, but tens of millions of markers, thus opening an even more precise window on the genetic history of roses. Note that the conservation mode has facilitated the task of studying the DNA of these historic roses directly from the current plants kept in pink. Indeed, thanks to grafting, the varieties of roses are potentially immortal!
Thanks to this analysis, we were first able to confirm the results of previous studies which, although based on a limited number of genetic markers, had already highlighted that the diversification of the 19th century rosese century resulted in successive crossings between ancient European roses and Asian roses. The high resolution offered by genomics, however, allowed us to go further: we have shown that this diversity was in reality built on a very reduced number of generations of crosses, recurring in a recurring way of flagship varieties of the time, used as parents in many crosses. It is remarkable to note that this happened with a good dose of chance (via pollination) since artificial fertilization (choice of two parents of the crossing) is only used on the rose from the middle of the 19th century.
Although based on a limited number of generations of crosses, contributing to a mixture between Asian and European roses, our study also made it possible to show that roses have significant genetic diversity. However, the selection carried out during the XIXe century has contributed to a slight erosion of this diversity, especially among varieties from the end of the century. However, maintaining a large genetic diversity is essential for resilience and adaptation of species in the face of environmental changes. Its long -term preservation therefore represents a major issue. As long as the old varieties are kept, this loss remains reversible. It is therefore crucial to act to avoid their final disappearance by preserving the collections of old and botanical roses.
On a complete genome scale, selection tends to reduce genetic diversity. But on a finer scale, its effects can be even more marked, resulting in a much more pronounced local decrease in diversity. Our study thus revealed that a region of chromosome 3, containing different forms of a key gene involved in the rise in flowering, has been the subject of a particularly intense selection in the XIXe century. This result, although predictable given the importance of this character, was clearly confirmed in the light of genomic data. More unexpectedly, we have also identified other regions of the genome with similar signatures of selection, in particular on chromosomes 1, 5 and 7. At this stage, the genes concerned, and the possible associated morphological characters still remain to be identified. Despite the advances of the genomics, the marriage of the Western and the East roses in the XIXe century still keeps a number of its family secrets!
With an unwavering passion for local news, Christopher leads our editorial team with integrity and dedication. With over 20 years’ experience, he is the backbone of Wouldsayso, ensuring that we stay true to our mission to inform.
